ON CRABS WHICH CLING. 181 



centralisation, is seen in the form of the antennnles or first 

 feelers and their flicking motion, hut none of the true crabs 

 possess in any appreciable degree the habit of clinging to the 

 under sides of stones, and, besides, none retain Galathea's 

 power of swimming backwards by flapping the abdomen. I 

 should mention here the well-known fact that the swimming- 

 crab ( Guernesiais "Lady Crab ") has regained to some extent 

 the power of swimming, for which purpose, however, it uses 

 the flattened last pair of walking legs ; the abdomen had gone 

 too far on the downward road to regain its old activity. 



A comparison between Galathea and the Crabs is of 

 interest for its own sake and for the glimpse it gives us of the 

 evolution of the latter, through a Dromia-like form, from a 

 lobster-type which must have shared a good many common 

 characters and habits with Galathea. 



Perhaps it is not too S}ieculative to regard the broadening 

 of the body in all the types we are discussing as due in part 

 to the broadening of the gill cavity. A narrow gill cavity 

 would be in danger of getting blocked with sand and mud, as 

 the gills would almost com})letely All it. Research by Weldon 

 with the shore crab. Caret nus iMceiias^ has affVjrded in this 

 connection the first approach to a real experimental demon- 

 stration of natural selection. He noticed a continuous in- 

 crease of breadth of crabs' backs on a certain stretch of shore 

 in Plymouth Sound, w^hich w as receiving increasing quantities 

 of mud through deflections of a current by extension of the 

 breakwater. He therefore gathered numbers of crabs, placed 

 them in his tanks and, when they had recovered from the 

 change, made the water more and more muddy. The effect 

 always was that a number of crabs died, but amongst the dead 

 the narrow-backed ones always formed the great majority. 



We must, however, return and emphasize the contrast 

 between the ordinary crabs and that common little descendant 

 of Galathea known as the Mud Crab or Porcelain Crab — 

 PorceUana^ of which genus the species lAutyclieh^ is 

 common everywhere. It is true that Porcellana is in many 

 ways a much less highly evolved type than the crabs, but in 

 spite of his seeming close resemblance to them he seems to 

 avoid competition to some extent by a marked divergence of 

 habit. The ordinary crab lives on the floor of sea or shore, 

 often imder stones it is true, but still walking right side up. 

 Porcellana has specialised by developing the clinging habit of 

 (yalathea and almost entirely discarding the other attitudes of 

 that ancestral type. With the clinging habit w^e saw w^ere 

 associated, in Galathea, the short body and the arranging of 



